1. Field
Embodiments of the disclosure relate to an organic light emitting diode (OLED) display driven in a digital driving manner.
2. Related Art
Various flat panel displays whose weight and size are smaller than cathode ray tubes have been recently developed. Examples of the flat panel displays include a liquid crystal display (LCD), a field emission display (FED), a plasma display panel (PDP), and an electroluminescence device.
Because the PDP has a simple structure and is manufactured through a simple process, the PDP has been considered as a display device having characteristics such as lightness in weight and thin profile and providing the large-sized screen. However, the PDP has disadvantages such as low light emitting efficiency, low luminance, and high power consumption. A thin film transistor (TFT) LCD using a TFT as a switching element is the most widely used flat panel display. However, because the TFT LCD is not a self-emission display, the TFT LCD has a narrow viewing angle and a low response speed. The electroluminescence device is classified into an inorganic light emitting diode display and an organic light emitting diode (OLED) display depending on a material of an emitting layer. Because the OLED display is a self-emission display, the OLED display has characteristics such as a fast response speed, a high light emitting efficiency, a high luminance, and a wide viewing angle.
The OLED display, as shown in FIG. 1, includes an organic light emitting diode. The organic light emitting diode includes organic compound layers between an anode electrode and a cathode electrode. The organic compound layers include a hole injection layer HIL, a hole transport layer HTL, an emitting layer EML, an electron transport layer ETL, and an electron injection layer EIL. When a driving voltage is applied to the anode electrode and the cathode electrode, holes passing through the hole transport layer HTL and electrons passing through the electron transport layer ETL move to the emitting layer EML and form an exciton. Hence, the emitting layer EML generates visible light.
The OLED display may be classified into an analog type OLED display and a digital type OLED display depending on a driving manner of the OLED display. In the analog type OLED display, as shown in FIG. 2A, a gray scale is represented by applying a data voltage, whose a magnitude varies, or a data current, whose a magnitude varies, to pixels. In the digital type OLED display, as shown in FIG. 2B, a gray scale is represented by changing an application time of a data voltage having a constant magnitude or a data current having a constant magnitude.
In the analog type OLED display, electrical characteristics (for example, threshold voltage, electron mobility, etc.) of a drive TFT, that controls an amount of current flowing in the organic light emitting diode depending on the magnitude of the data voltage or the data current, change depending on driving time or process conditions in each pixel. Therefore, it is difficult to accurately represent the gray scale in the analog type OLED display. In the digital type OLED display, because a drive TFT is used as only a switching element, a reduction in image quality resulting from a difference between electrical characteristics of a drive TFT can be prevented.
In the general digital type OLED display, video data corresponding to 1 frame is divided into j bit-planes (where j is a integer equal to or greater than 2), and 1 frame is time-divided into k subfields (where k is a integer equal to or greater than 2), so that the OLED display displays the video data during 1 frame period. Each of the j bit-planes is assigned to one subfield or the plurality of subfields.
In the digital type OLED display, it looks like a subfield of a previous frame and a subfield of a current frame overlap because of a difference between an integral direction of light and visual characteristics of the light perceived through the human eye. As a result, a dynamic false contour noise in which a luminance is distorted may occur. The dynamic false contour noise is generally measured in the form of white band or black band. In particular, the dynamic false contour noise remarkably occurs when gray levels (for example, 127-128, 63-64, and 31-32), whose light emitting patterns are greatly different from each other, are successively represented. For example, as shown in FIG. 3, when a value of gray level changes from 31 to 32, a brightness difference between two frames is 1. However, first to fifth subfields T1 to T5 are turned on so as to represent 31-gray level, and a sixth subfield T6 is turned on so as to represent 32-gray level. When the gray level changes from 31 to 32, a moving amount of a light emitting point increases because of a large time lag between light emitting patterns in the two frames. As a result, the dynamic false contour noise occurs.
The time lag between the light emitting patterns in the two frames allows a driving frequency F2 perceived through the human eye to be smaller than an original driving frequency F1 and thus may cause a screen flicker.